Carbonates (e.g., calcium carbonate) have been widely used in various fields including rubber, plastic, and paper. In recent years high-performance carbonates have been increasingly developed and are used for many purposes according to their specific features such as particle shape and diameter.
Calcite, aragonite and vaterite are the examples of the crystalline forms of carbonate. Of the three, aragonite is useful in many applications because it is composed of needle-shaped particles and thereby offers excellent strength and elastic modulus.
For example, generally known processes for carbonate production are: a process in which carbonate ion-containing solution is reacted with chloride-containing solution; and a process in which a chloride is reacted with carbonic acid gas. In addition, there are processes that are proposed for the production of needle-shaped carbonates with an aragonite-like structure: a process in which carbonate ion-containing solution is reacted with chloride-containing solution under application of ultrasonic wave (see Patent Literature 1); and a process for introducing carbon dioxide into aqueous slurry of Ca(OH)2, in which the process a needle-shaped aragonite crystal is previously introduced in the aqueous slurry as a seed crystal followed by growing of the seed crystal in a given direction (see Patent Literature 2).
The production process disclosed in Patent Literature 1, however, has a problem that its use results in the formation of large carbonate particles of 30-60 μm in length with a broader particle size distribution, which makes it impossible to produce carbonate particles with a desired, controlled size. Also, the use of the production process disclosed in Patent Literature 2 merely results in the formation of large carbonate particles of 20-30 μm in length.
Meanwhile, there is a strong tendency in recent years that polymer resins are increasingly used for general optical components (e.g., glass lenses and transparent plates) and optical components designed for optoelectronics, particularly for materials of optical components of laser-related devices used for instance in optical disc apparatus for recording of sounds, pictures, texts, etc. One of the reasons for this is that optical polymer materials (optical materials made of polymer resin) are generally excellent in terms of lightness, cost, formability and productivity compared to other optical materials such as optical glass. In addition, polymer resins are advantageous because molding techniques, such as extrusion molding or injection molding, can be readily applied.
However, molded articles formed from such conventional, general optical polymer material by means of any of the molding techniques are known to show birefringence. Although polymer materials that offer birefringence are not especially problematic when used in optical elements that do not require so high optical precision, there is a high demand in recent years for high-precision optical articles. For example, birefringence causes a serious problem in rewritable magneto optical discs. That is, since such a magneto optical disc utilizes a polarized beam as a reading or a recording beam, the presence of a birefringent element (e.g., the disc itself or a lens) in an optical path affects the precision of reading or recording of information.
To reduce the degree of birefringence to avoid this problem, there is proposed a non-birefringent optical resin material formed from inorganic particles and polymer resin, the birefringence values of which are of opposite sign (see Patent Literature 3). The optical resin material is prepared by the process called crystal doping. More specifically, a number of inorganic particles are dispersed in polymer resin, and a molding force is applied from the outside by drawing or the like, allowing linking chains present in the polymer resin and the inorganic particles to align in a direction that is substantially parallel to each other, so that the birefringence due to the optical anisotropy of the linking chains is canceled by the birefringence of the inorganic particles, which the value is the opposite sign.
In a case where a non-birefringent optical resin material is to be prepared by crystal doping, it is imperative to adopt inorganic particles that are applicable to crystal doping. It is recognized that fine carbonate particles that have needle or rod shapes are particularly suitable for such inorganic particles.    (Patent Literature 1) Japanese Patent Application Laid-Open (JP-A) No. 59-203728)    (Patent Literature 2) U.S. Pat. No. 5,164,172    (Patent Literature 3) International Publication No. WO 01/25364